The present invention relates to a spiral magnetic transmitter for position measurement system. The present invention is generally in the field relating to devices for measuring the position of receiving antennas with respect to transmitting antennas using magnetic fields. Some such systems are able to measure position and orientation in six degrees of freedom, including translative motion in the three coordinate directions x, y and z, and orientation in the three orientation coordinates pitch, roll and azimuth. The present invention is equally applicable to systems designed to measure in fewer or greater than six degrees of freedom.
In the prior art, it is known to use magnetically based position and orientation measuring systems in the fields of biomechanics and medical diagnostics. In those fields, a sensor assembly is mounted on a point of interest and the position of that point is determined relative to a fixed transmitter to precisely show the relative motions of the point in question. In the medical environment, such systems are used to precisely locate an instrument or other object within the human body, thereby permitting complex methods of surgery and diagnostics to be carried out where accuracy in location is essential. In such applications, it is advantageous to provide a transmitting means that is relatively small and does not interfere with the therapeutic procedure that is taking place.
It is also known to make magnetic transmitters that are flat. In the prior art, a plurality of dipole or non-dipole transmitters are arranged conveniently to produce a field pattern that is useful in determining object position. However, when the sensor approaches the plane of the transmitter loops, position and orientation measurement become difficult as the vectors from the transmitter loops become highly parallel.
Dipole systems also suffer from the phenomenon of magnetic cross-coupling, as the magnetic field from one dipole element induces current in another adjacent element which induced current is then re-radiated by that element, thereby producing undesirable field distortions. Methods of preventing or eliminating such re-radiations are known in the art but require increased complexity in transmitter driver circuitry. Prior art non-dipole systems also suffer from this re-radiation effect. Additionally, in the case of the prior art non-dipole systems, the shape of the equi-potential surface of the vector magnitude also changes from spherical to toric as distance from the transmitter loops decreases. This phenomenon causes numerous difficulties when attempting to determine position and/or orientation of an object within a prescribed space.
The following prior art is known to Applicant:
U.S. Pat. No. 5,600,330 to Blood discloses a non-dipole loop transmitter based magnetic tracking system which utilizes formed elongate conductor patterns which describe a loop. In the Blood system, when the sensor approaches the plane of the transmitter loops, position determination becomes difficult and unstable as the vectors from the transmitter loops become parallel. The shape of the equi-potential surface of the vector magnitude also changes from spherical to toric as distance from the transmitter loops decreases. This causes numerous difficulties in finding a position and/or orientation solution. Blood fails to teach or suggest a method of finding position and orientation using a planar spiral antenna and such an antenna is nowhere disclosed by Blood.
U.S. Pat. No. 5,752,513 to Acker et al. discloses a method and apparatus for determining position of an object. Acker et al. disclose use of an elongated thin conductor as the transmitting element and various antenna configurations. All of those configurations are effectively formed from the equivalent of elongate loop conductors with the current carrying components of the transmitter means residing substantially along the outer boundary of the transmitter means. Acker et al. disclose overlapping adjacent transmitter loops but the methods disclosed by Acker et al. inherently result in significant cross-coupling of the coil axes. Acker et al. fail to teach or suggest any method of reducing the undesirable cross-coupling that occurs.
U.S. Pat. No. 5,198,768 to Keren discloses a surface coil array for use in nuclear magnetic resonance applications. Keren fails to determine position nor does Keren describe methods of reducing cross-coupling between adjacent coils.
U.S. Pat. No. 5,640,170 to Anderson discloses a transmitter configuration employing a spiral conductor pattern over a conductive ground plane in order to produce a low distortion dipole field over the configuration. The Anderson device cannot be used in a planar configuration as it requires that a conventional two axis dipole be located above the spiral or that multiple spirals intersect at right angles. Either one of these options results in an increased transmitter profile. Anderson discloses that operation of the Anderson system results from a new previously undiscovered principle of boundary condition behavior but fails to disclose methods to minimize cross-coupling effects from additional axes which may be employed by the system.
Additionally, the present invention represents a distinct departure from the prior art known to Applicant relating to transmitting and receiving position and orientation devices since the present invention is capable of satisfying the requirement of operation near the plane of the transmitter and operates using conventional boundary condition physics while not requiring special current densities or conductive planes. The present invention operates with either dipole or non-dipole transmitted fields. Additionally, a method of construction is disclosed which results in the ability to arrange up to three transmitter coils which overlap but are free from undesirable cross-coupling.